76 DH. J. W. JEXKIXSON ON THE [Feb. 6, 



principal ones accessoiy cotyledons of smaller size are formed in 

 the intervening regions throughout the later peiiods of gestation. 

 The exact mode of their formation presents some points of 

 interest. 



The process has been thus described by Kolster : " Die erste 

 Anlage einer solchen accessoi-ischen Placenta aussei-t sich darin 

 dass eine nicht all zu weite wohl aus einer einfachen Chorionzotte 

 hervorgehenden Epitheltasche in die subepitheliale Kernschicht 

 hineinwachst," With this account I am obliged, in two respects, 

 to disagree : in the first place the new crypt is not formed vmder 

 the influence of an ingrowing villus, and in the second it does 

 not, in the first instance, grow down into the subepithelial tissue. 

 By carefully preserving maternal and foetal tissues together, the 

 exact mode of origin of the structures can be made out witliout 

 difl[iculty (text-figs. 27 a-e). 



The first sign of such an accessory crypt (in the Cow) is to be 

 fovnid in the pitting of the viterine epithelium, the underlying 

 subepithelial tissue remaining perfectly smooth and taking no 

 part in the process. The cells which line the bottom of these pits 

 are shorter, the cells which make their sides rather taller than 

 the cells of the surrounding unmodified epithelium. It may be 

 noticed that mitoses are frequently met with in the cells of these 

 pits, though rai'e in other parts of the epithelium (text-figs. 27 «, c). 



Pi-esently, the under layer of connective tissue still I'emaining 

 pei'fectly smooth, a few wandering cells make their way through 

 into the walls of the pit (text-fig. 27 b), and this is soon followed 

 either by the formation of connective fibres around these 

 wandering cells or by the extrusion of fibres from the layer below 

 (text-fig. 27 c). A small pitted area — with connective tissue and 

 capillaries in the walls of the pits — thus becomes raised above 

 the general level of the uterine surface (text-fig. 27 c) and by a 

 continuation of the process a cotyledon is formed. Only after the 

 pits have attained a considerable degree of development do the 

 foetal villi begin to dip into them ; that this is the case is very 

 clearly shown by the fact that while the pits are being formed 

 the overlying trophoblast is absolutely smooth and non-villous. 



The cells of the pits, like those of the general epithelium, are 

 at first ciliated (though this is denied by Kolster) and more or less 



Explanation of Text- fig. 27 (opposite). 



[Note. — Text-tigs. 27, 28, 29, & 31 are from drawings made with the Camera 

 Liicida, obj. Zeiss 2 mm. achr., oc. comp. 4.] 



Details of the formation of an accessory cotyledon in a Cow's placenta of 

 the 6th month. 



Fi^. a. A small epithelial pit is formed ; fig. h, the wall separating two pits is 

 growing up above the level of the surrounding epithelium. Ihe underlying 

 connective-tissue layer is at present quite undisturbed ; fig. c, the connective 

 tissue begins to grow up into the pit or crypt-wall. Notice the mitoses. 



Fig. d shows the somewhat cubical but still ciliated epithelium lining a crypt in 

 one of these newly formed accessory cotyledons ; and fig. e the continuity of the 

 ciliated with the modified vacuolated epithelium of a rather older crypt. 



